Author ORCID Identifier

https://orcid.org/0000-0002-7064-865X

Date Available

8-2-2021

Year of Publication

2021

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department/School/Program

Mechanical Engineering

First Advisor

Dr. Lyndon Scott Stephens

Abstract

PTFE-based materials are widely used in areas of tribology, particularly in seal and bearing applications because of their outstanding self-lubricating properties. Often in dynamic seal applications there is a need for ultra-low mechanical friction loss between the sealing surfaces. Due to its extremely low friction coefficient, there is interest in employing Polytetrafluoroethylene (PTFE) materials in such applications. One challenging aspect of employing PTFE is that these materials are viscoelastic and plastic. This dissertation concentrates on the modeling of viscoelastic material response when used as mechanical face seals with a focus on PTFE-based materials. First, the viscoelastic characteristics are measured through experimental tests. Using a dynamic mechanical analyzer (DMA), the storage modulus, loss modulus and tan δ, are measured and discussed in the frequency domain. The relaxation modulus and creep compliances are also measured and studied in the time domain. Furthermore, the materials’ compositions are studied using Energy Dispersive X-ray Spectrometer for composites measurement. The experimental data is then modeled with best fit curves using the Prony series and compared to the experimental data.

In seals made of viscoelastic materials, harmonic oscillations can lead to separation of seal faces and leakage. The isothermal viscoelastic dynamic response of a PTFE end face seal subjected to small harmonic input and preload static displacement from an ideally rigid opposing face is examined. Both the magnitude and time of separation of the faces are predicted based on dry conditions (no lubrication effect due to leakage). The presented model is a hybrid that combines the Golla-Hughes-McTavish (GHM) finite element model, a delayed recovery creep model and a penalty method contact model. The GHM and delayed recovery creep models are first validated using experimental data from a Dynamic Mechanical Analyzer (DMA) test for PTFE-based materials. Results for a simple sample application show seal separation magnitude as a function of frequency and applied harmonic displacement amplitude due to vibration of the rigid face. Results show that face separation occurs in PTFE seals even for small amplitude harmonic vibrations (as compared to the preload static displacement) and that this is due to the viscoelastic damping effect creating a phase lag between the motions of the faces. A simple leakage estimate is also performed for the sample application.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2021.337

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